There are many applications which require fast switching speeds. For example, for multi-mode multi-band cell phone applications such as GSM (Global System for Mobile Communications), GPRS (General Packet Radio Service), and 3G (Third Generation Wireless), the antenna switch unit switches the antenna to different bands as well as between transmission (TX) and receiving (RX) modes. Currently, solid-state switches are used for this purpose. While RF (Radio Frequency) MEMS metal contact series switches generally have much better insertion loss and isolation characteristics, they are much slower than solid-state switches.
Referring to FIGS. 1A and 1B, these figures illustrate a side view and a top view of a MEMS in-line cantilever beam metal contact series switch, respectively. This type of MEMs switch can be manufactured by well known MEMS fabrication processes.
As shown, the switch is formed on a substrate 100. A metalized signal line 102 may be formed on one side of the substrate 100 and a second signal line 104 may be formed on the second side of the substrate 100. A cantilevered beam 106 may be secured to the second signal line 104. A bump (electrode) 108 may be formed on the underside of the cantilevered beam 106 over the first signal line 102. An actuation plate 110 may be formed on the substrate 100 beneath the cantilevered beam 106. When the actuation plate 110 is energized, by applying a voltage on the actuation lead 112, the cantilevered beam 106 is pulled downward causing the bump 108 to make electrical contact with the first signal line 102. This closes the switch and provides an electrical signal path between the first signal line 102 and the second signal line 104.
For Tx/Rx switching, speeds of a few micro-seconds are typically needed. To reach such speeds for MEMS switches, the switch structure (i.e., the cantilevered beam 106) should preferably be very stiff so the mechanical resonance frequency is high. This also means the actuation voltage required for the switch is higher (40-100V) to overcome the stiffness. In such cases, high voltage driver chips may be required. Such driver chips may be fabricated using special CMOS processes to achieve this activation voltage. These are often expensive and add to the total cost of the switch module.